Automatic overload protection



Nov. 2, 1954 T. P. KINN AUTOMATIC OVERLOAD PROTECTION Filed Feb. 8, 1951 INVENTOR Theodore P Kinn.

ATTORNEY United States Patent 2,693,537 AUTOMATIC OVERLOAD PROTECTION Theodore P. Kinn, Baltimore, Md., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application February 8, 1951, Serial No. 210,009 Claims. (Cl. 250-36) My invention relates to radio frequency heating apparatus, and in particular to a control circuit for the radio frequency power source which governs the power output of said power source.

In accordance with the prior art control apparatus with which I am familiar, when the load increases, the voltage across the tank circuit of the power source decreases to cause, by means of prior control apparatus, the filament voltage of the oscillator tube thereof to rise. The increased filament voltage results in an increased power being fed to the load. As the load power increases above a predetermined limit, the prior art control apparatus operates to shut down the whole heating operation. This results in lost production, especially when the control apparatus shuts down equipment such as a conveyor line assembly feeding workpieces of dielectric load material to the radio frequency heating apparatus, in the manner of a mass production assembly. When the heating operation is reinstated, the overload protective equipment will subsequently shut down the operation every time an excessive plate load occurs.

Accordingly, it is an object of my invention to provide an overload protective apparatus which will eifectively limit the peak power supplied to the load.

It is another object of my invention to avoid unnecessary lost production resulting from the overload protective equipment shutting down the heating operation every time an excessive plate load occurs.

It is still another object of my invention to limit the power source oscillator plate current to a safe value without the interruption of the heating operation.

It is a further object of my invention to provide control apparatus for radio frequency heating apparatus which will provide safe operation of the heating apparatus for all loads from zero to overload.

It is a still further object of my invention to provide control apparatus for dielectric heating equipment which will allow continuous heat treatment of all loads.

My invention arises from the realization of a need for an overload protective system which will function to prevent the plate current of a radio frequency power source oscillator tube from going too high, when the effective load being fed by said power source increases to the point where the plate current becomes unreasonable in value. As the load effectively increases, to drop the tank voltage due to the usual poor regulation in load circuits of this type, the prior art control apparatus in response to the change in tank voltage caused the filament voltage to rise in an effort to maintain a suitable tank voltage, and hence increased the power being fed into the load. As the tank voltage dropped, said control apparatus responded to increase the filament voltage and effectively increase the plate current of the supply oscillator tube, in an effort to raise the tank voltage to its normal value.

My invention includes the provision of a current responsive means, such as a potentiometer, in the plate circuit of the oscillator tube to respond to the plate current. A voltage which is proportional to the plate current is taken from this potentiometer, and applied in the negative relation to the control apparatus for otherwise controlling the filament voltage in response to a drop in the tank voltage. In this manner, I limit the maximum load demand by using the filament voltage control to reduce filament voltage to a value which will limit the maximum plate current of the oscillator tube to a predetermined maximum safe level.

The novel features that I consider characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and its method of operation, together with additional objects and advantages thereof, will be understood from the following description of the specific embodiments thereof, when read in connection with the accompanying drawing in which the single figure is a schematic diagram of one form of apparatus and circuits embodying my invention, particularly as applied to dielectric heating.

Referring to the single figure, the apparatus shown includes a radio frequency power supply generator 10 including an oscillator tube 12, which has a plate 14, a control electrode 16 and a cathode 18. A tank circuit 22 which includes a tank inductance 24 and plate by-pass condenser 26, which are connected in parallel with a tank condenser 20. One side of the tank condenser 20 is connected to the plate 14 of the oscillator tube 12, and the other side is connected to ground. The required plate voltage for the oscillator tube 12 is obtained from an alternating current source 28 through a transformer 30 and a plate voltage supply rectifier 32. An R-F choke coil 34 is in series with the rectifier 32 to prevent the passage of radio frequency currents from the tank circuit 22 to the rectifier 32. Suitable load electrodes may be coupled to the tank circuit 22 in any conventional manner, such as inductive coupling to inductance coil 24, as well known to those skilled in the art. Also, in some applications the tank condenser 20 may be, if desired, used as the load heating member.

The cathode 18 of the oscillator tube 12 is supplied filament power from the alternating current source 28 through an adjustable autotransformer 36 and a stepdown filament transformer 38. The autotransformer 36 is controlled by a reversible motor 40.

The control grid 16 is connected to limiting resistor 44, a T-network filter 46 bias determining resistor 48; the latter resistor 48 is shunted by a by-pass condenser 50. In parallel with the cathode resistor 42 is a variable potentiometer 52, the adjustable arm 54 of which is connected through a first magnetic winding 56 of a balanced relay 57, an R. F. choke coil 58 and a condenser 72 to ground.

A tank voltage rectifier circuit 60 is connected to the junction between a pair of voltage dividing condensers 62 and 64, the latter condenser 64 being connected to the output tank circuit 22, and condenser 62 being connected to ground. The rectifier network includes a rectifier diode 66 having a cathode resistor 68 connected in series with it, the latter resistor 68 being connected to the adjustable arm 54 of potentiometer 52. A load resistor 70 is provided for the diode 66.

The second magnetic winding 74 of the balanced relay 57 is connected to a constant reference voltage source 76. The latter constant reference voltage source 76 includes a rectifier bridge 78 connected through a transformer 80 to the alternating current power source 28. The output of the rectifier bridge 78 is connected through a 1r filter section 82 and a pair of adjustable resistors 84 and 86 to the second winding 74 of the balanced relay 57. A voltage regulator tube 88 shunts the output of the constant reference voltage source 76.

A second bridge circuit 90 is connected across the output of the supply transformer 80, the output of said second rectifier bridge 90 being connected across a potentiometer 92 with a filter capacitor 94 shunting said potentiometer 92. The variable arm 96 on said potentiometer 92 is connected to the cathodes 98 and 100 of a pair of control switch tubes 102 and 104 respectively. Each of said control switch tubes 102 and 104 comprises a tetrode having its screen grids 106 and 108 connected to its cathodes 98 and 100 respectively. A control switch tube plate supply voltage transformer 110 has its input connected to the alternating current power supply source 28 and its output connected to the respective plates 112 and 114 of the two control switch tubes 102 and 104 through respective plate load resistors 116 and 118 and respective relay members 120 and 122. The first control switch tube 102 is connected to control the operation of a first relay member 120, said first relay member 120 operating contact member 124 to bridge a pair of contacts and effectively control the reversible motor 40 to increase the filamentvoltage supplied to the oscillator tube 12 through the autotransformer 36. A spark-suppression condenser 126 shunts the magnetic winding of said first relay member 120. The second control switch tube 104 is connected to energize the magnetic winding of the second relay member 122 to operate its contact member 128 to bridge a second pair of contacts to operate the reversible motor 40 in a direction to reduce the filament voltage supplied to said oscillator tube 12 through the filament supply autotransformer 36. A spark-suppression condenser 130 shunts the magnetic winding of the second relay member 122.

The polarized relay 57 has a movable contact arm 132 which is connected to one end 134 of the secondary winding 136 of a transformer 138, which has its primary winding 140 connected to the alternating current power source 28. The other end 142 of said secondary Winding 136 is grounded. This movable contact arm 132 is positioned to move between a first and a second contact 144 and 146 respectively. The first contact 144 is connected to apply a control voltage to the control grid 107 of the first control switch tube 102 through a grid resistor 148. The second contact 146 is connected to apply control voltage to the control grid 109 of the second control switch tube 104 through a grid resistor A voltage dropping resistor 152 and 154 respectively is provided across the secondary winding 136 of said transformer 138 for each of the balanced relay contact positions 144 and 146.

The cathode circuit of tank voltage monitoring diode tube 66 is returned through the resistor 68 to the adjustable arm 54 of potentiometer 52 and thereby to ground.

In the operation of the apparatus shown in Fig. 1, the tank circuit 22 determines the oscillation frequency of the oscillator tank 12. The first bridge circuit 78 provides a substantially constant reference voltage to energize the second magnetic winding '7 4 of the polarized relay member 57. The first winding 56 of the polarized relay member 57 is energized, to effectively balance the relay 57, by a voltage which is obtained through the rectifier tube 66 from the voltage of the tank circuit 22, in combination with a voltage which is obtained from the potentiometer 52 shunting the cathode resistor 42 placed in the cathode circuit of the oscillator tube 12, and which latter resistor 42 is responsive to the plate current of the oscillator tube 12. The second rectifier bridge circuit 90 provides a reference potential which determines the cathode voltage level and hence the operation of the two control switch tubes 102 and 104. The plate voltage for said two control switch tubes 102 and 104 is provided through the control switch tube plate voltage supply transformer 110. Each of the latter control switch tubes 102 and 104 respectively governs the operation of the reversible motor in either a first or a second direction. The latter reversible motor 40 accordingly controls the adjustment of a filament supply autotransformer 36 which, in turn, determines the filament voltage for the oscillator tube 12.

Under normal load conditions, the polarized relay 57 is balanced, and the control switch tubes 102 and 104 are not functioning to result in the reversible motor 40 not being energized. Therefore, the filament voltage is at its initial value as required for the normal operation of the oscillator tube 12.

The effective load reflected into the oscillator tank circuit 22 by any of the normal methods such as inductive coupling to tank coil 24, under normal operating conditions, varies considerably. If the effective load increases due to a change in the heating effect on a workpiece between the electrodes comprising a dielectric load, or by a change in the physical size of the material between the heating electrodes, the voltage of the tank circuit 22 will drop, due to the imperfect regulation common with all load circuits. This drop in tank voltage will be detected by the rectifier circuit coupled to' the tank circuit 22, and will result in the energization of the first magnetic winding 56 of the balanced relay member 57 decreasing in amount, to result in the second winding 74 of the balanced relay member 57 being effectively energized to a greater amount by the constant reference voltage source 76. This will result in the balanced relay member 57 changing its position, from its normal neutral position, to a position such that its movable arm 132 contacts the first contact 144 and hence energizes the control grid 107 of the first control switch tube 102. The first control switch tube 102, when it operates, will energize the winding on the first relay member 120, controlling the reversible motor 40, and cause the latter motor 40 to operate in a direction to increase the filament voltage being applied to the oscillator tube 12. The increased filament voltage will result in the oscillator tube 12 attempting to increase the voltage of the tank circuit 22.

However, the latter increased filament voltage will also result in the plate current of the oscillator tube 12 increasing. The potentiometer 52 which shunts the cathode resistor 42 will respond to this increase in plate current, and will effectively oppose the operation of the rectifier circuit 60 to deenergize the first magnetic winding 56 of the balanced relay member 57. In this manner, the operation of the rectifier circuit 60 to increase the tank voltage is opposed by the operation of the shunt potentiometer 52 to limit the plate current, and hence the tank voltage and plate current values are limited to values which are safe respecting the operation of the oscillator tube 12 and system in general.

If the effective load on the oscillator tube 12 decreases, the tank voltage will increase, due to the imperfect regulation common to all load circuits of this type, and the rectifier circuit 60 will effectively increase the energization of the first magnetic winding 56 of the balanced relay member 57 to change the position of the movable arm 132 to contactthe second contact 146. When the latter second contact 146 is contacted in this manner, the control grid 109 of the second control switch tube 104 is energized to cause the second control switch tube 104 to function. When the second control switch tube 104 functions, the second relay member 122 in the plate circuit of the latter tube 104 is energized to operate the reversible motor 40 in a direction to reduce the filament voltage applied to the oscillator tube 12 by means of the filament supply autotransformer 36. When the filament voltage is reduced in this manner, the oscillator tube 12 operates to reduce the voltage of the output tank circuit 22 and thus maintain a maximum safe value of no load tank voltage.

When the filament voltage applied to the oscillator tube 12 is reduced in this manner, this also effectively reduces the plate current of the oscillator tube 12, and the potentiometer 52 shunting the cathode resistor 42 is of such a value that the voltage which is obtained from said potentiometer 52 does not substantially interfere withthe operation of the rectifier circuit 60 to energize the first magnetic winding 56 of the balanced relay 57.

Accordingly, my control circuit operates to limit the plate current of the radio frequency power source oscillator tube 12 when the load increases to overload values, which are above the safe operating conditions of the tube 12 and system. However, when the effective load decreases, and hencethe plate current of tube 12 decreases, my control circuit does not interfere with the operation of that portion of the over-all control apparatus which responds to the decrease in loading to effectively degresze the power output of the power supply oscillator tu 6 Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

i claim as my invention:

1. In a control circuit for a radio frequency power source, said power source including at least one oscillator tube having a plate circuit and a filament circuit, with a control voltage applied to said filament circuit and a tank circuit connected to the plate circuit, said tank circuit being adapted for supplying power to a dielectric load, the combination of a balanced relay member for varying said control voltage, a first voltage supply source connected to said relay member for providing a substantially constant reference first voltage to said member, and a second voltage supply source connected to said relay member for providing a second voltage proportional to the v current in the plate circuit of said tube to therelay' thember, said second voltage supply source includinga potentiometer in the plate circuit of said tube.

age effects the position of said relay in a negative relation respecting the effect of said reference first voltage on said relay member.

3. In a control circuit for a radio frequency power source adapted for the heat treatment of a load, the combination of a first voltage supply source for providing a substantially constant reference voltage, a second voltage supply source for providing a second voltage which is proportional to the voltage of said load, said power source including an oscillator tube having a plate circuit and a filament circuit, said second voltage supply source including a potentiometer connected in the plate circuit of said oscillator tube, and a control member connected to said power source by filament circuit.

4. In a control circuit for a radio frequency generator including an oscillator tube having a tank circuit for member being each of said reference, second and third voltages to determine said control voltage.

5. The apparatus of claim 4 characterized by said control means being connected to compare each of said second and third voltages with the reference voltage.

6. The apparatus of claim 4 characterized by said conrol means being connected such that the second and third voltages are each compared with said reference voltage, with said third voltage affecting said control means in a negative manner relative to the effect of said second voltage.

7. The apparatus of claim 4 characterized by said control means including a balanced relay member having a plurality of arms, with said reference voltage affecting one or" the relay member arms and the second and third voltages affecting the second relay member arm.

8. The apparatus of claim 4 characterized by said controlling means including a polarized relay member having at least two arms, with said reference voltage being operative to effect the movement of one of said arms and the second and third voltages being operable to effect the movement of the second arm.

9. The apparatus of claim 1 with a third voltage supply means including a rectifier for providing a third voltage proportional to the tank circuit voltage, with said control means being responsive to said third voltage, and with the effect of said third voltage on said control means being opposite to the effect of said second voltage on said control means.

10. The apparatus of claim 9, with said potentiometer having a resistance value such that the effect of said second voltage is less than that of the third voltage on said control means when the load decreases below its normal value.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,149,080 Wollf Feb. 28, 1939 2,175,694 Jones Oct. 10, 1939 2,236,195 McKesson Mar. 25, 1941 2,415,799 Reifel Feb. 11, 1947 2,416,172 Gregory Feb. 18, 1947 2,545,997 Hagopian Mar. 20, 1951 2,587,175 Lappin Feb. 26, 1952 

